The present invention relates generally to lighting sources for photographic imaging applications, and more particularly to computer-controlled lighting sources for vision systems used in testing of products, components, or sub-assemblies.
In modern manufacturing environments, vision systems are often used to test products, components, or sub-assemblies in order to verify that the item has been properly manufactured or assembled. A vision system performs, in an automated way, a visual inspection analogous to that which would otherwise be done by a person. Among other benefits, the vision system can relieve a human operator of the tedium associated with such an inspection, thus freeing him to perform other functions, and can automatically maintain quality records and analyze trends so as alert the manufacturing personnel of actual or potential quality problems with the manufactured item.
Vision systems typically utilize a video camera to capture and digitize images of the item being tested, and a computer connected to the video camera to analyze the digitized images and compare them to what is expected to be observed. For the vision system to operate properlyxe2x80x94that is, to correctly identify tested items as being either good or badxe2x80x94each item being tested must be appropriately illuminated for the vision test being performed. If items are illuminated improperly, the vision system may incorrectly identify a good item as a bad one, and reject it. Incorrectly rejecting a significant number of good items results in increased manufacturing costs which ultimately get passed on to the consumer.
In order to minimize the number of items which are incorrectly rejected by the vision system as non-complying, once the appropriate level of illumination for a particular test is determined, the light source should be capable of providing a certain constant light output (or intensity) for desired intervals over a long period of time. Furthermore, if a varying amount of ambient light can also illuminate the item during testing, the light source should be capable of both detecting this ambient illumination and varying its light output so as to maintain the total illumination on the item at a constant level.
Certain vision applications require the light source to provide light of different intensities during different tests, or for testing different items. For example, two identical items supplied from different vendors or produced from two different manufacturing runs may have different reflectivity, requiring different illumination levels for the vision system to operate properly. Or, in one test the vision system may view the item through a protective wrapping that requires more illumination, and in another test without the wrapping that requires less illumination.
Other vision applications require the light source to provide different color lighting. For example, a first color light may be used to view surface features on a partially-transparent window of an item, while a second color light is used to view the internal features of the item through the window.
In addition, to view surface features of an item that project in a direction towards the video camera lens, it is advantageous for the light source to provide side, rather than frontal, illumination. Providing side illumination to these surface features can generate shadows which are more easily detected by the vision system. Also, when illuminating highly reflective items, side illumination may reduce certain reflections which can interfere with proper vision system operation.
Some previous light sources use halogen or incandescent lighting, which often are subject to fading over time. In addition, such light sources tend to have relatively short lifetimes, requiring frequent replacement of the light sources. These light sources also tend to generate excessive heat which often requires cooling so as not to affect the item being tested or other elements of the vision system, thus resulting in added cost or complexity of the light source. While fluorescent light sources tend not to produce excessive heat, fading over time is still a problem. With any of the above light sources, filters are required in order to provide different color light output. Such filters are expensive, not easily changed, and provide only a limited number of discrete light colors. Other previous vision system light sources use light-emitting diodes of a certain color. Where the intensity of any of these light sources is selectable, it is typically done manually.
Accordingly, the need still exists for a light source that can produce a specified light output of a specified color, can maintain a constant light output for desired intervals over a long period of time, and can automatically adjust the light output so as to maintain a constant illumination level on an object under a variety of ambient lighting situations.
In a preferred embodiment, the present invention may be implemented as a novel lighting arrangement that produces a constant light output of a specified intensity, and of a specified hue and direction, over a long lifetime, and can automatically adjust the light output in the presence of ambient light in order to maintain a contain illumination on the object being illuminated. Such a lighting arrangement can be advantageously used in vision system applications in manufacturing and inspection, where consistent lighting of the objects to be tested or inspected under all ambient lighting conditions is essential to accurate operation of the vision system and avoiding incorrect rejection of conforming objects.
The lighting arrangement includes a light source which generates a predetermined light output in response to a light control signal provided by a light controller which is electrically connected to the light source. The light source is mounted in a housing which, during operation, is positioned near the object to be illuminated. The light controller has an input connectable to the vision system so that the vision system can specify to the light controller the predetermined light output to be provided by the light source. The light source has a plurality of lighting elements mounted in the housing. The lighting elements are preferably each controllable independently. While the housing can take a variety of shapes, a preferred embodiment is a toroidally-shaped housing where the lighting elements are arranged on one of the planar surfaces of the housing so as to evenly illuminate an object positioned adjacent that planar surface. The housing may include a diffuser mounted between the lighting elements and the object. At least some of the lighting elements of the preferred embodiment are light-emitting diodes. These diodes may all generate light of essentially the same color or hue, or at least some of the light-emitting diodes emit light of a different hue from at least some others of the light-emitting diodes; in addition, by varying the intensity of light emitted from the different color diodes, the hue of the light output can be varied. Alternatively, light-emitting diodes that emit light of a variable hue can be used in other embodiments. The intensity of the light emitted by each light-emitting diode is controlled by pulse width modulation. All the diodes may be pulse-width modulated in the same way so as to produce light of the same intensity, or different pulse width modulation signal may be applied to different light-emitting diodes so that different diodes emit light of a different intensity. By varying the intensity of light output based on the position of the light-emitting diodes in the housing, the directionality of the illumination on the object may be varied.
Another embodiment of the present invention is a lighting arrangement which provides a predetermined illumination on an object which is illuminated by a varying ambient light as well as light supplied from the light source. The lighting arrangement includes an illumination sensor positioned near the object which detects the total illumination on the object resulting from both the supplied light and the ambient light. The sensor produces an illumination signal proportional to the total illumination, and this signal is electrically transmitted to the light controller. A negative feedback circuit in the light controller varies the light control signal in response to the illumination signal so as to maintain the predetermined illumination on the object. The desired level of predetermined illumination can be externally specified to the lighting arrangement, such as by a vision system, via a programming interface on the light controller. Alternatively, the predetermined illumination level can be set by manual controls on the lighting arrangement. Typically, the light sensor is mounted in the same housing containing the lighting elements.
Yet another embodiment of the present invention is a method for illuminating an object with a predetermined illumination. The method includes specifying the predetermined illumination desired, and applying light of an initial intensity level derived from the predetermined illumination. The total illumination on the object resulting from applying the initial intensity level light is sensed, and then a corrected intensity level based on the initial intensity level set and the total illumination sensed is determined. Then light of the corrected intensity level is reapplied so as to illuminate the object with the desired illumination. Typically the object is also illuminated by ambient light as well as the light supplied from the lighting arrangement, and so the total illumination sensed includes the ambient light illumination on the object as well as the supplied light illumination. Preferentially, in order to compensate for fluctuations in the ambient light illumination, the total illumination is periodically sensed, and a new corrected intensity level determined and reapplied, so as to maintain the predetermined illumination.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.